Latching mechanism with adjustable preload

ABSTRACT

A well pipe hanger is run and landed within a subsea wellhead assembly using a latching mechanism with an adjustable preload. The well pipe hanger has a latch ring having upward and downward facing shoulders secured to the outer diameter of the tubing hanger so that the latch ring may expand radially into a groove of the landing sub. The outer and inner diameters of the latch ring are larger at a center of the latch ring than at the upper and lower ends. The latch ring is configured to contract radially from a latched to an unlatched position in response to a predetermined overpull force. The predetermined overpull force is adjustable by adjusting the radial distance between an axial center of the latch ring and the outer diameter of the well pipe hanger.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates in general to well pipe hangers and, in particular, to a casing hanger having a latching mechanism with an adjustable preload.

2. Brief Description of Related Art

A typical subsea wellhead assembly includes a wellhead housing that supports one or more casing hangers. One type of wellhead housing has a conical load shoulder machined within its bore. The casing hanger lands on and is supported by the load shoulder. In this type, the diameter of the housing bore below the shoulder is less than the diameter of the housing above the shoulder by a dimension equal to a radial width of the load shoulder.

In another type, referred to as “full bore”, the wellhead housing has an annular groove within the bore with substantially the same diameter above and below the groove. The load shoulder may be an annular member on the casing hanger designed to expand into the groove. The casing hanger is supported by the load shoulder. This procedure allows a larger diameter bore to be employed during drilling operations. In these embodiments, the load shoulder may be installed on a special running tool or it may be run with the casing hanger.

Active casing hangers may be used to transfer the casing load to the wellhead housing via a loading mechanism that includes an activation ring, shear pins that prevent premature movement of the activation ring, and a load ring on the casing hanger. This mechanism is typically designed to be activated by the weight of the string when a reaction point, such as a shoulder, formed on the interior of the wellhead housing is reached during lowering of the hanger. At this point, the shear pins on the activation ring break to allow the activation ring to slide relative to the downward movement of the hanger, thereby allowing the load ring on the hanger to align with the housing to transfer the casing load to the housing. This also increases the bearing area of the casing hanger. However, if the hanger snags or the pins load up unevenly and break prematurely, the activation ring may be activated prematurely. This is costly and time consuming as the hanger and casing would have to be pulled out and re-tripped.

Wellheads may include multiple landing grooves for multiple casing strings or other equipment. When running a particular casing string, it is important to ensure that the casing hanger lands in the appropriate load shoulder. Some systems rely on an operator's knowledge of the depth of the well and the length of the running string supporting the casing hanger. These systems require the operator to land the casing hanger and, based on the operator's estimation, estimate that the length of the landing string was sufficient to place the casing hanger on the correct shoulder. Other systems employ mechanical or electrical systems that provide positive confirmation of correct landing of the casing hanger within the correct wellhead shoulder. These systems may employ complicated equipment that requires sufficient capital expenditures to modify the casing hanger, run additional lines or umbilicals to the wellhead, and provide an interfacing means for the operator to receive the positive indication of landing. As a consequence, they are not widely used. Thus, there is a need for a cost effective method to ensure that a casing hanger and casing string have landed on the appropriate shoulder within a wellhead.

SUMMARY OF THE INVENTION

These and other problems are generally solved or circumvented, and technical advantages are generally achieved, by preferred embodiments of the present invention that provide a casing hanger having a latching mechanism with an adjustable preload, and a method for using the same.

In accordance with an embodiment of the present invention, a subsea wellhead assembly is disclosed. The subsea wellhead assembly includes a subsea wellhead member having a central bore defining an annular landing shoulder and an annular groove formed within an inner diameter of the bore and spaced below the landing shoulder. The subsea wellhead assembly also includes a well pipe hanger having an inner and outer diameter, a central axis, and a load shoulder for landing on the landing shoulder of the well member. A latch ring having upward and downward facing shoulders is carried on the outer diameter of the well pipe hanger. The latch ring is radially resilient so that the latch ring snaps radially into the groove of the wellhead member when the load shoulder lands on the landing shoulder. The latch ring is configured to contract radially from a latched to an unlatched position in response to a predetermined overpull force, allowing the load shoulder of the well pipe hanger to rise above the landing shoulder. The latch ring is free to snap back into the groove when the well pipe hanger is lowered back onto the landing shoulder.

In accordance with another embodiment of the present invention, a subsea wellhead assembly is disclosed. The subsea wellhead assembly includes a subsea wellhead member having a central bore defining an annular landing shoulder and an annular landing groove formed within an inner diameter of the bore and spaced below the landing shoulder. The subsea wellhead assembly includes a casing hanger having an inner and outer diameter, a central axis, and a load shoulder for landing in a load shoulder of the landing sub. A latch ring having upward and downward facing shoulders is secured to the outer diameter of the casing hanger so that the latch ring may expand radially into a latched position in the groove of the subsea wellhead. The latch ring is configured to contract radially from a latched to an unlatched position in response to a predetermined overpull force. The predetermined overpull force is adjustable by adjusting a radial distance between an axial center of the latch ring and the outer diameter of the casing hanger. An outer diameter of the latch ring is larger at a center of the latch ring than at the upper and lower ends. Similarly, an inner diameter of the latch ring is larger at the center of the latch ring than at the upper and lower ends. The latch ring defines a plurality of slots spaced equidistantly around the latch ring so that the latch ring may expand and contract radially.

In accordance with yet another embodiment of the present invention, a method for confirming landing of a well pipe hanger in a subsea wellhead member is disclosed. The method provides, in a bore of the wellhead member, a landing shoulder and an annular groove spaced below the landing shoulder and provides a well pipe hanger having a radially moveable latch ring. The method runs the well pipe hanger into the wellhead member and lands the well pipe hanger on the landing shoulder. The method allows the latch ring to contact an inner diameter wall of the bore and radially contract as the well pipe hanger enters the bore, then radially expand to latch within the groove. The method then applies an upward axial force to the well pipe hanger, causing the latch ring to contract and release from the groove, thereby lifting the well pipe hanger from the landing shoulder. Then, the method lowers the well pipe hanger back onto the load shoulder and allows the latch ring to radially expand back into engagement with the groove.

An advantage of a preferred embodiment is that it provides a casing hanger landing confirmation system using overpull. The casing hanger includes an adjustable mechanism that allows the overpull load required to test the casing hanger to be adjusted to accommodate the conditions of any one particular wellhead load shoulder. In addition, the disclosed embodiments provide a casing hanger with a latching mechanism that is reusable that does not require refurbishing of the casing hanger with subsequently perishable components such as shear elements.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the features, advantages and objects of the invention, as well as others which will become apparent, are attained, and can be understood in more detail, more particular description of the invention briefly summarized above may be had by reference to the embodiments thereof which are illustrated in the appended drawings that form a part of this specification. It is to be noted, however, that the drawings illustrate only a preferred embodiment of the invention and are therefore not to be considered limiting of its scope as the invention may admit to other equally effective embodiments.

FIG. 1 is a sectional view of a casing hanger latched within a landing sub in accordance with an embodiment of the present invention.

FIG. 2 is a sectional view of a portion of the casing hanger and latching mechanism of FIG. 1.

FIG. 3 is a perspective view of a latch ring in accordance with an embodiment of the present invention.

FIG. 4 is a sectional view of the portion of the casing hanger and latching mechanism of FIG. 2 latched within the landing sub of FIG. 1.

FIG. 5A is a sectional view of the portion of the casing hanger and latching mechanism of FIG. 2 latched during an overpull test of FIG. 1.

FIG. 5B is a sectional view of the portion of the casing hanger and latching mechanism of FIG. 2 within the landing sub of FIG. 1 following an overpull test.

FIG. 6 is a sectional view of a portion of the casing hanger and latching mechanism of FIG. 2 with the latching mechanism in a free state.

FIG. 7 is a sectional view of an alternative embodiment of the present invention.

FIG. 8 is a sectional view of an alternative embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention will now be described more fully hereinafter with reference to the accompanying drawings which illustrate embodiments of the invention. This invention may, however, be embodied in many different forms and should not be construed as limited to the illustrated embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout, and the prime notation, if used, indicates similar elements in alternative embodiments.

In the following discussion, numerous specific details are set forth to provide a thorough understanding of the present invention. However, it will be obvious to those skilled in the art that the present invention may be practiced without such specific details. Additionally, for the most part, details concerning rig operations, wellbore drilling, wellhead placement, and the like have been omitted inasmuch as such details are not considered necessary to obtain a complete understanding of the present invention, and are considered to be within the skills of persons skilled in the relevant art.

Referring to FIG. 1, there is shown a landing sub 11, such as a wellhead, high pressure housing, tubing hanger spool, or the like. Landing sub 11 may include a bypass channel 13, as shown. Bypass channel 13 may allow fluid to flow through an annulus between landing sub 11 and a casing hanger 15 disposed within landing sub 11. After the casing string is cemented in the well, a pack-off or seal seals between casing hanger 15 and landing sub 11, closing off bypass channel 13. In the illustrated embodiment, casing hanger 15 defines a central bore 17 having an axis 19. Casing hanger 15 may include an overpull latching assembly 21 secured to an outer diameter of casing hanger 15. Overpull latching assembly 21 will latch within a landing sub annular groove 23 formed in an inner diameter of landing sub 11. In the illustrated embodiment, casing hanger 15 includes an annular protrusion 31 formed on the outer diameter of casing hanger 15 axially above overpull latching assembly 21.

Referring to FIG. 2, overpull latching assembly 21 is shown secured to casing hanger 15. Overpull latching assembly 21 may include a retainer ring 25, a latch ring 27, and a preload ring 29. Annular protrusion 31 may define a downward facing shoulder 33. Casing hanger 15 may also include a thread 35 formed on an lower exterior diameter of casing hanger 15. Retainer ring 25 includes an upper end 37 having a surface adapted to abut downward facing shoulder 33. Retainer ring 25 further includes an annular portion 39 extending axially downward from an outer diameter of retainer ring 25. The lower end of casing hanger 15 will be inserted through retainer ring 25 so that upper end 37 of retainer ring 25 will abut downward facing shoulder 33. Once assembled, annular portion 39 of retainer ring 25 will define an upper annular cavity 41 between annular portion 39 and casing hanger 15. Retainer ring 25 further defines a downwardly facing load shoulder 26 on an outer diameter portion of retainer ring 25 adapted to land on a landing shoulder 30 as shown in FIG. 4.

Latch ring 27 includes an upper end 43 and a lower end 45. Upper end 43 may insert into upper annular cavity 41 between annular portion 39 and casing hanger 15. Retainer ring 25 may secure upper end 43 of latch ring 27 to casing hanger 15 while allowing latch ring 27 to expand axially within upper annular cavity 41. In addition, upper end 43 may abut against an end of upper annular cavity 41, limiting the total upward axial expansion of latch ring 27 and the total axial movement of latch ring 27 relative to casing hanger 15. Latch ring 27 also includes a latching protrusion 47. Preferably, latching protrusion 47 extends from an outer diameter center portion of latch ring 27 approximately equidistant from upper end 43 and lower end 45. Latching protrusion 47 may define an upwardly facing shoulder 49 and a downwardly facing shoulder 51. Preferably, upwardly and downwardly facing shoulder 49, 51 may insert into landing sub annular groove 23 (FIG. 1) of landing sub 11. Upwardly and downwardly facing shoulders 49, 51 may also be tapered as illustrated so that latching protrusion 47 may slide out of landing sub annular groove 23 under appropriate conditions.

As illustrated, the inner and outer diameters of latch ring 27 at latching protrusion 47 are greater than the inner and outer diameters of latch ring 27 at upper end 43 and lower end 45. Thus, the inner and outer diameters of latch ring 27 slope radially outward from upper end 43 to latching protrusion 47 and from lower end 45 to latching protrusion 47. Preferably, the slope between upper end 43 and latching protrusion 47 is substantially equivalent to the slope between lower end 45 and latching protrusion 47, although a person skilled in the art will understand that these slopes may be different or vary between the points.

Preload ring 29 defines a preload thread 53 on an inner diameter lower end of preload ring 29. Preload thread 53 may match thread 35 on the outer diameter of the lower end of casing hanger 15 so that preload ring 29 may secure to casing hanger 15 through matching threads 35, 53. Preload ring 29 further includes an annular portion 55 extending axially upward from an outer diameter of preload ring 29. Preload ring 29 will be threaded onto casing hanger 15 so that annular portion 55 of preload ring 29 will define a lower annular cavity 57 between annular portion 55 and casing hanger 15.

Lower end 45 of latch ring 27 may be inserted into lower annular cavity 57 as preload ring 29 is threaded onto casing hanger 15. Inner diameter distal surfaces of annular portions 39, 55 of retainer ring 25 and preload ring 29, respectively, are tapered to match the slope between upper and lower ends 43, 45 and latching protrusion 47 of latch ring 27. As preload ring 29 is threaded through matching threads 35, 53, the tapered inner surfaces of annular portions 39, 55 may contact the exterior diameter of latch ring 27. Further threading of preload ring 29 through matching threads 35, 53 will move upper and lower ends 43, 45 of latch ring 27 into upper annular cavity 41 and lower annular cavity 57, respectively. During initial assembly of latching mechanism 21 to casing hanger 15, a gap 28 will be formed between the inner diameter of latch ring 27 at latching protrusion 47 and the outer diameter of casing hanger 15. Preferably, latching protrusion 47 will extend past the outer diameters of casing hanger 15, retainer ring 25, and preload ring 29, allowing latching protrusion 47 to insert into landing sub annular groove 23 of landing sub 11. As upper and lower ends 43, 45 of latch ring 27 move further into upper and lower annular cavities 41, 57, the tapered surfaces will cause a resilient deflection of latch ring 27 radially inward toward the outer diameter of casing hanger 15 while expanding latch ring 27 axially. Adjustment of preload ring 29 by threading preload ring 29 more or less through matching threads 35, 53 will adjust the amount of inward radial deflection of latch ring 27, increasing or decreasing the size of gap 28. In this manner, a desired overpull may be selected for latch ring 27, as described in more detail below.

Referring to FIG. 3, latch ring 27 defines a plurality of slots 59. Slots 59 may pass radially through latch ring 27 and may extend between lower end 45 and upper end 43 such that latch ring 27 defines a continuous ring at upper and lower ends 43, 45, respectively. Preferably, slots 59 may be spaced equidistantly around latch ring 27 so that each slot is equidistant from the adjacent slots 59. Similarly, each slot 59 will have a similar size and shape so that slots 59 are generally uniform. In this manner, slots 59 will define a plurality of latching members 61 similarly spaced around latch ring 27. Preferably, each latching member 61 extends from lower end 45 to upper end 43, is of a similar size and shape, and includes a latching protrusion 47. As described above, each latching member 61 has an inner diameter at upper and lower ends 43, 45 that is smaller than the inner diameter at latching protrusion 47. In the illustrated embodiment, the inner diameter at upper and lower ends 43, 45 is approximately equal to the outer diameter of casing hanger 15 (FIG. 2), allowing for latch ring 27 to circumscribe casing hanger 15.

Prior to run in of casing hanger 15, preload ring 29 is adjusted so that latch ring 27 will deflect radially inward to release latching protrusion 47 from landing sub annular groove 23 at the desired predetermined overpull load. The preload amount is determined by the amount of overpull desired to release casing hanger 15 from landing sub 11. This is based in part on the necessary force required to deflect latch ring 27 radially inward. This, in turn, is based on the material properties of latch ring 27, the size and number of latching members 61, the amount of deflection imparted to latch ring 27 by preload ring 29 prior to run in of casing hanger 15, and the angle of the abutting shoulders 49, 61. To set a desired overpull load, preload ring 29 is threaded through matching threads 35, 53 to cause either more or less preload deflection of latch ring 27. In turn, this causes the size of gap 28 to vary so that latching protrusion 47 may displace more or less into landing sub annular groove 23. In addition, by varying the number and size of slots 59, latch ring 57 may provide varying resistance to deflection, in turn, varying the needed overpull load.

Referring to FIG. 4, overpull latching assembly 21 is assembled to casing hanger 15. Latch ring 27 has been preloaded a predetermined amount by threading of preload ring 29 to casing hanger 15 to cause a radial inward deflection of latch ring 27. Casing hanger 15 may then be run to landing sub 11 in a manner known to those skilled in the art so that load shoulder 26 lands on landing shoulder 30, transferring the weight of casing hanger 15 and the attached casing string to landing sub 11.

During run in of casing hanger 15, the inner diameter of landing sub 11 may cause further radially inward displacement of latch ring 27. Preferably, the angle of shoulder 51 will allow a rim or shoulder of landing sub 11 to easily deflect latch ring 27 for run in of casing hanger 15. When casing hanger 15 reaches the appropriate location within landing sub 11, so that load shoulder 26 lands on landing shoulder 30, latching protrusion 47 of latch ring 27 will be proximate to landing sub annular groove 23 of landing sub 11. Latch ring 27 may then displace radially outward to expand into landing sub annular groove 23. As shown in FIG. 4, when load shoulder 26 of retainer ring 25 lands on landing shoulder 30, upwardly facing shoulder 49 may not be in contact with downwardly facing shoulder 61 of landing sub annular groove 23.

As shown in FIG. 5A, when an operator receives an indication that load shoulder 26 has landed on landing shoulder 30, an overpull test may be conducted. Casing hanger 15 will be pulled upwards axially so that shoulder 26 of retainer ring 25 and landing shoulder 30 are no longer in contact. Continued upwards axial movement will bring upwardly facing shoulder 49 of latching protrusion 47 into contact with downwardly facing shoulder 61 of landing sub annular groove 23. This will stop upward axial movement of casing hanger 15. Application of increasing upwards axial force to casing hanger 15 will continue until the upwards axial force reaches the predetermined overpull load. When the predetermined overpull load is reached, latch ring 27 will deflect radially inward so that latching protrusion 47, and consequently upwardly facing shoulder 49, is pulled from landing sub annular groove 23, as shown in FIG. 5B. Casing hanger 15 may then be moved upwards axially without additional resistance. An operator conducting the overpull test will understand that the lack of resistance indicates that latch ring 27 has been displaced and no longer resides within landing sub annular groove 23. When latch ring 27 displaces at the designed overpull load, this provides confirmation to the operator that casing hanger 15 has landed in the appropriate position within landing sub 11. Where latch ring 27 displaces at an overpull load other than the designed overpull load, this provides confirmation to the operator that casing hanger 15 did not land at the desired position within landing sub 11. The operator may then remove casing hanger 15 or run casing hanger 15 further axially downward through landing sub 11 to land at the desired location. Preferably, each landing shoulder 30 within landing sub 11 will cause latch ring 27 to deflect at a different overpull load.

Preferably, preload ring 29 will maintain the desired preload amount once reached. As shown in FIG. 6, preload ring 29 may be secured at the desired preload amount through use of set screws 67. Preload ring 29 may include threaded bores 65 placed at desired locations around preload ring 29. Preload ring 29 may then be assembled to casing hanger 15 as shown in FIG. 6 and described above. Once the desired preload amount is reached, set screws 67 may be threaded into threaded bores 65. Preferably, an end of each set screw 67 will contact and grip thread 35 of casing hanger 15. In this manner, undesired rotation of preload ring 29 during operational use of casing hanger 15 may be limited.

Referring to FIG. 7, an alternative latching assembly 69 is shown. Latching assembly 69 includes a latch ring 71 and a preload bolt 73. Latch ring 71 includes an upper end 75 and a lower end 77. Latch ring 71 also includes a latching protrusion 79. Preferably, latching protrusion 79 extends from an outer diameter center portion of latch ring 71 approximately equidistant from upper end 75 and lower end 77. Latching protrusion 79 may define an upwardly facing shoulder 81 and a downwardly facing shoulder 83. Preferably, upwardly and downwardly facing shoulder 81, 83 may insert into landing sub annular groove 23 of landing sub 11. Upwardly and downwardly facing shoulders 81, 83 may also be tapered as illustrated so that latching protrusion 79 may slide out of landing sub annular groove 23 under appropriate conditions.

As illustrated, the inner and outer diameter of latch ring 71 at latching protrusion 79 is greater than the inner and outer diameter of latch ring 71 at upper end 75 and lower end 77. Thus, the inner and outer diameters of latch ring 71 slope from upper end 75 to latching protrusion 79 and from latching protrusion 79 to lower end 77. Preferably, the slope between upper end 75 and latching protrusion 79 is substantially equivalent to the slope between latching protrusion 79 and lower end 77, although a person skilled in the art will understand that these slopes may be different or vary between the points. A bore 85 is formed near a center of latching protrusion 79 extending through a radial width of latch ring 71. Preload bolt 73 may be inserted into bore 85 and threaded into a corresponding threaded bore 87 formed in an outer diameter portion of casing hanger 15. Preload bolt 73 may be threaded into threaded bore 87 by varying amounts to adjust the radial distance from an outer diameter of casing hanger 15 to an outer diameter of latching protrusion 79. In this manner, the desired overpull amount is adjusted by varying the height of latching protrusion 79. In the illustrated embodiment, latch ring 71 includes a preload limiter 89. Preload limiter 89 may be an annular boss formed on an inner diameter portion of latch ring 71 near bore 83 and opposite latching protrusion 79. Preload limiter 89 may contact an outer diameter of casing hanger 15 when preload bolt 73 is threaded into threaded bore 87 a predetermined amount.

Referring to FIG. 8, in another alternative embodiment, latch ring 71 may be fitted into a latch ring housing 91. Latch ring 71 will include all of the components of latch ring 71 of FIG. 7. Latch ring housing 91 may be a ring having an inner diameter thread 93 corresponding to the inner diameter thread 35 of casing hanger 15, allowing latch ring housing 91 to be threaded onto a lower end of casing hanger 15 in a manner similar to preload ring 29 of FIG. 2. As shown in FIG. 8, latch ring housing 91 will define a latch ring cavity 95. Latch ring cavity 95 will have an annular channel opening 97 on an outer diameter portion of latch ring housing 91. In this manner, latch ring 71 may be inserted into latch ring cavity 95 so that latching protrusion 79 may pass from latch ring cavity 95 to an exterior of latch ring housing 91 through channel opening 97. Similar to the embodiment of FIG. 7, the embodiment of FIG. 8 may include preload bolt 73 (not shown) passing through a bore 85 of latch ring 71. Preload bolt 73 will thread into a threaded bore (not shown) of latch ring housing 91. Preload bolt 73 may be threaded into threaded bore in varying amounts to adjust the radial distance from a surface of latch ring cavity 95 to an outer diameter of latching protrusion 79. In this manner, the desired overpull amount is adjusted by varying the height of latching protrusion 79. In the illustrated embodiment, latch ring 71 includes a preload limiter 89. Preload limiter 89 may be a boss formed on an inner diameter portion of latch ring 71 near bore 83 and opposite latching protrusion 79. Preload limiter 89 may contact the surface of latch ring cavity 95 when preload bolt 73 is threaded into threaded bore 87 a predetermined amount.

Accordingly, the disclosed embodiments provide numerous advantages over the prior art. For example, the disclosed embodiments provide a latching mechanism that allows an operator to preselect the overpull load necessary to test each particular load shoulder or groove. In addition, the disclosed embodiments allow the operator to vary this overpull amount as needed based on the conditions of the well, rather than forcing the operator to rely on an apparatus ordered and delivered prior to the operator's experience with that particular well and operating conditions. Still further, the disclosed embodiments, provide a latching mechanism that provides a separate overpull check mechanism that avoids the risk of activation of a load ring before desired as in prior art embodiments. Still further, the disclosed embodiments are assembled and operated without any sacrificial parts such as shear elements or the like. This allows the disclosed embodiments to be reused on both a single well and multiple wells with only minor adjustments to the assembly, namely the adjustment of the preload amount by rotation of the preload ring or preload bolt.

It is understood that the present invention may take many forms and embodiments. Accordingly, several variations may be made in the foregoing without departing from the spirit or scope of the invention. Having thus described the present invention by reference to certain of its preferred embodiments, it is noted that the embodiments disclosed are illustrative rather than limiting in nature and that a wide range of variations, modifications, changes, and substitutions are contemplated in the foregoing disclosure and, in some instances, some features of the present invention may be employed without a corresponding use of the other features. Many such variations and modifications may be considered obvious and desirable by those skilled in the art based upon a review of the foregoing description of preferred embodiments. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention. 

What is claimed is:
 1. A subsea wellhead assembly comprising: a subsea wellhead member having a central bore defining an annular landing shoulder and an annular groove formed within an inner diameter of the bore and spaced below the landing shoulder; a well pipe hanger having an inner and outer diameter, a central axis, and a load shoulder for landing on the landing shoulder of the well member; a latch ring having upward and downward facing shoulders, the latch ring being carried on the outer diameter of the well pipe hanger, the latch ring being radially resilient so that the latch ring snaps radially into the groove of the wellhead member when the load shoulder lands on the landing shoulder; wherein the latch ring is configured to contract radially from a latched to an unlatched position in response to a predetermined overpull force, allowing the load shoulder of the well pipe hanger to rise above the landing shoulder; and the latch ring is free to snap back into the groove when the well pipe hanger is lowered back onto the landing shoulder.
 2. The subsea wellhead assembly of claim 1, wherein: the outer diameter of the latch ring is larger at a center of the latch ring than at the upper and lower ends; the inner diameter of the latch ring is larger at the center of the latch ring than at the upper and lower ends; and the latch ring defines a plurality of slots spaced equidistantly around the latch ring so that the latch ring may expand and contract radially.
 3. The subsea wellhead assembly of claim 2, wherein the predetermined overpull force is adjustable by adjusting the radial distance between an axial center of the latch ring and the outer diameter of the tubing hanger.
 4. The subsea wellhead assembly of claim 2, wherein the well pipe hanger further comprises: a retainer ring secured to an outer diameter of the well pipe hanger; wherein an upper end of the latch ring is interposed in an annular space between the retainer ring and the well pipe hanger; a well pipe hanger thread formed on a lower outer diameter surface of the well pipe hanger; a preload ring having an inner diameter preload thread matching the well pipe hanger thread; wherein the preload ring threads onto the well pipe hanger thread so that a lower end of the latch ring is interposed in an annular space between the preload ring and the well pipe hanger; and tightening the preload ring decreases the amount the latch ring protrudes from the well pipe hanger and increases an axial length of the latch ring, thereby adjusting the overpull force.
 5. The subsea wellhead assembly of claim 4, wherein: the well pipe hanger thread has a length longer than a length of the preload thread; the preload ring may move an axial distance along the well pipe hanger by rotating through the preload thread; an inner diameter portion of the preload ring and the retainer ring contact an outer diameter portion of the latch ring; and wherein moving the preload ring axially through the well pipe hanger thread and preload ring thread causes the latch ring to contract and expand radially.
 6. The subsea wellhead assembly of claim 5, wherein radial displacement of the latch ring corresponds to the predetermined axial overpull force.
 7. The subsea wellhead assembly of claim 5, wherein at least one set screw threads through the preload ring and into the tubing hanger threads to limit rotation of the preload ring.
 8. The subsea wellhead assembly of claim 1, wherein the well pipe hanger further comprises: a plurality of threaded bore holes formed in the outer diameter of the well pipe hanger, wherein the threaded bore holes are positioned at the same axial height and spaced so that each bore hole is equidistant from adjacent bore holes; the latch ring defines a plurality of bores passing radially through the latch ring, each bore of the latch ring aligning with a threaded bore of the well pipe hanger; a plurality of bolts pass through the latch ring bores to thread into the well pipe hanger bores, thereby securing the latch ring to the well pipe hanger; and the plurality of bolts threaded into the threaded bore holes to preload the latch ring by radially displacing the latch ring.
 9. The subsea wellhead assembly of claim 1, wherein the well pipe hanger further comprises: a latch ring housing having an inner and outer diameter; a well pipe hanger thread is formed on a lower outer diameter surface of the well pipe hanger; the latch ring housing having an inner diameter thread matching the well pipe hanger thread, wherein the latch ring housing threads onto the well pipe hanger thread; a plurality of threaded bore holes are formed in the outer diameter of the latch ring housing, wherein the threaded bore holes are positioned at the same axial height and spaced so that each bore hole is equidistant from the adjacent bore holes; the latch ring defines a plurality of bores passing radially through the latch ring, each bore of the latch ring aligning with a threaded bore of the latch ring housing; a plurality of bolts pass through the latch ring bores to thread into the latch ring housing bores, thereby securing the latch ring to the latch ring housing; and wherein the plurality of bolts thread into the threaded bore holes so that the latch ring is displaced radially a predetermined amount corresponding with a predetermined axial overpull amount.
 10. The tubing hanger of claim 9, wherein rotating the latch ring housing moves the latch ring to a predetermined axial height relative to the well pipe hanger.
 11. A subsea wellhead assembly comprising: a subsea wellhead member having a central bore defining an annular landing shoulder and an annular landing groove formed within an inner diameter of the bore and spaced below the landing shoulder; a casing hanger having an inner and outer diameter, a central axis, and a load shoulder for landing in a load shoulder of the landing sub; a latch ring having upward and downward facing shoulders secured to the outer diameter of the casing hanger so that the latch ring may expand radially into a latched position in the groove of the subsea wellhead; wherein the latch ring is configured to contract radially from a latched to an unlatched position in response to a predetermined overpull force; wherein the predetermined overpull force is adjustable by adjusting a radial distance between an axial center of the latch ring and the outer diameter of the casing hanger; wherein an outer diameter of the latch ring is larger at a center of the latch ring than at the upper and lower ends; wherein an inner diameter of the latch ring is larger at the center of the latch ring than at the upper and lower ends; and wherein the latch ring defines a plurality of slots spaced equidistantly around the latch ring so that the latch ring may expand and contract radially.
 12. The subsea wellhead assembly of claim 11, wherein the casing hanger further comprises: an annular landing protrusion formed around an outer diameter of the casing hanger, wherein the landing protrusion defines a downward facing shoulder; a retainer ring secured to an outer diameter of the casing hanger axially beneath and abutting the downward facing shoulder of the landing protrusion; wherein an upper end of the latch ring is interposed between the retainer ring and the casing hanger; a preload ring mounted to the casing hanger by threads; wherein the preload ring may move an axial distance along the casing hanger by rotating through the preload ring relative to the casing hanger; wherein an inner diameter portion of the preload ring and the retainer ring contact outer diameter portions of the latch ring; and wherein moving the preload ring axially causes the latch ring to contract and expand radially.
 13. The subsea wellhead assembly of claim 12, wherein: the larger inner and outer diameters at the center of the latch ring than at the ends of the latch ring define a latch ring protrusion that bulges outward from the latch ring; and applying a preload force with the preload ring reduces the extent of the bulge.
 14. The subsea wellhead assembly of claim 12, wherein: the retainer ring further comprises an annular portion extending axially toward the preload ring, wherein an inner diameter surface of the annular portion defines a taper in contact with an outer diameter surface of the latch ring above the latch ring upward facing shoulder; and the preload ring further comprises and annular portion extending axially toward the retainer ring, wherein an inner diameter surface of the annular portion defines a taper in contact with an outer diameter surface of the latch ring below the latch ring downward facing shoulder.
 15. The subsea wellhead assembly of claim 11, wherein: the casing hanger defines a plurality of threaded bore holes formed in the outer diameter of the casing hanger, wherein the threaded bore holes are positioned at the same axial height and spaced so that each bore hole is equidistant from the adjacent bore holes; the latch ring defines a plurality of bores passing radially through the latch ring, each bore of the latch ring aligning with a threaded bore of the casing hanger; a plurality of bolts pass through the latch ring bores to thread into the casing hanger bores, thereby securing the latch ring to the casing hanger; and wherein the plurality of bolts threaded into the threaded bore holes such that the latch ring may displace radially to adjust a predetermined overpull amount.
 16. The subsea wellhead assembly of claim 11, wherein the casing hanger further comprises: a latch ring housing having an inner and outer diameter; a casing hanger thread formed on a lower outer diameter surface of the casing hanger; the latch ring housing having an inner diameter thread matching the casing hanger thread so that the latch ring housing threads onto the casing hanger thread, securing the latch ring housing to the casing hanger; the latch ring housing defines a plurality of threaded bore holes formed in the outer diameter of the latch ring housing, wherein the threaded bore holes are positioned at the same axial height and spaced so that each bore hole is equidistant from the adjacent bore holes; the latch ring defines a plurality of bores passing radially through the latch ring, each bore of the latch ring aligning with a threaded bore of the latch ring housing; a plurality of bolts pass through the latch ring bores to thread into the latch ring housing bores, thereby securing the latch ring to the latch ring housing; and the plurality of bolts threaded into the threaded bore holes such that the latch ring may displace radially to adjust to a predetermined overpull force.
 17. The subsea wellhead assembly of claim 11, wherein upwards pull on the casing hanger at the predetermined load will cause the latch ring to compress radially, allowing the casing hanger to be pulled from the subsea wellhead.
 18. A method for confirming landing of a well pipe hanger in a subsea wellhead member, the method comprising: (a) providing in a bore of the wellhead member a landing shoulder and an annular groove spaced below the landing shoulder; (b) providing a well pipe hanger having a radially moveable latch ring; (c) running the well pipe hanger into the wellhead member and landing the well pipe hanger on the landing shoulder; (d) allowing the latch ring to contact an inner diameter wall of the bore and radially contract as the well pipe hanger enters the bore, then radially expand to latch within the groove; (e) applying an upward axial force to the well pipe hanger, causing the latch ring to contract and release from the groove, thereby lifting the well pipe hanger from the landing shoulder; then (e) lowering the well pipe hanger back onto the load shoulder and allowing the latch ring to radially expand back into engagement with the groove.
 19. The method of claim 18, wherein the method further comprises adjusting a preload ring prior to running of the well pipe hanger to adjust the predetermined upward axial force needed to cause a radial displacement of the latch ring. 